1. Filed of the Invention
This invention relates to a sewing machine in general and more particularly to an upper thread supplying device employed in the sewing machine, especially an electronically controlled sewing machine.
2. Description of the Prior Art
In a sewing machine operation, an upper thread is drawn out of a bobbin mounted in a machine housing to be supplied to a stitching needle. There has recently been provided means for supplying the upper thread, which is actuated in synchronism with reciprocation of the needle to draw a specific amount of the upper thread for producing a stitch, while the upper thread is not tensioned by an upper thread take-up lever. The stitching conditions including a fabric feeding amount, a needle amplitude and a fabric thickness will be inputted to a control unit or CPU, mounted in the machine housing, for theoretically determining an amount of the upper thread required for producing the stitch in accordance with a prescribed program. Such stitching conditions are selected by manipulation of select keys or detected by sensor means before or during the stitching operation.
However, it is often the case that the upper thread amount thus theoretically determined is not coincident with an amount actually required for producing the stitch. Such discrepancy would be caused by other stitching conditions not included in the data to be inputted to CPU for determining the theoretical amount. For example, a friction coefficient of a fabric to be sewn with respect to fabric feeding means or a feed dog will have an influence on fabric feeding efficiency. Should the fabric be made of a slippery material having a small friction coefficient, there would be a discrepancy between the theoretically determined amount and the actually required amount. The fabric thickness would be changed with a pressure applied to the fabric by means of a presser foot. A sliding resistance of the fabric and/or thread employed and expansion of the thread would also be fluctuation factors. These stitching conditions would be difficult to be processed into data and therefore have not been considered in determination of the upper thread amount to be supplied for producing the stitch. Consequently, even if the upper thread amount should be theoretically determined by conventionally inputted data regarding the stitching condition typically including the fabric feeding amount, the needle amplitude and the fabric thickness, there would arise an excessive supply or a shortage of the upper thread due to other stitching conditions not inputted as fluctuation data to CPU, resulting in deformation of the stitch formed on the fabric.